The present invention generally relates to the measurement of surface characteristics and, more particularly, to a method of and an apparatus for measuring surface characteristics, especially the surface glossiness and the surface roughness, of a workpiece such as a coating or a metal.
The glossiness and the surface roughness are one of the important factors determinative of surface characteristics of a workpiece and, in evaluating the finish of particularly a coating on the surface of the workpiece, the both are considered important characteristics to be taken into consideration. JIS (Japanese Industrial Standards)-Z-8741 stipulates the methods for the measurement of glossiness and generally defines two types of glossiness, the physically determinable glossiness including the specular glossiness and the contrast glossiness, and the psychologically determinable glossiness including the distinctness-of-image glossiness. As compared with the specular and contrast glossinesses, the distinctness-of-image glossiness is still based on the visual inspection. According to the JIS-Z-8741, the distinctness-of-image glossiness is defined as meaning the extent to which the image of an object reflects on the surface to be tested.
Based on this definition, various instruments for the measurement of the distinctness-of-image glossiness have hitherto deviced. One of them is disclosed in, for example, the Japanese Utility Model Publication No. 41-19039 published in 1966. The measuring instrument disclosed in the above mentioned publication is effective to present, in the form of a numerical value, the extent to which the image of each of character patterns of different size reflected on the surface to be tested can be discerned. However, since it has been found that the measured values differ from person to person and that the resolution of the measured values is not good, the measuring apparatus has not been used so often.
Another example disclosed in, for example, the Japanese Laid-open Patent Publication No. 50-153979, laid open to public inspection in 1975, discloses a method comprising causing the image of a pattern having sharply divided light and dark areas to be reflected from the surface to be tested, focusing the reflected image on a focal plane, and presenting, in the form of a numerical value, any change in intensity of light received from the light and dark areas of the focused image of the pattern. However, this known method has some disadvantages in that the measured value is directly affected by change in intensity of the light received due to change in reflectance of the surface to be tested, and in that, since the pattern of distribution of the intensity of light received does not represent a monotonous decrease affected by characteristics, such as roughness, of the surface to be tested, a lot of difficulties are involved in signal processing necessary to present the distinctness-of-image glossiness in the form of a numerical value based on the rate of change in intensity of the light received.
The Japanese Laid-open Patent publication No. 52-138960, laid open to public inspection in 1977, discloses a distinctness-of-image glossmeter of a construction as shown in FIG. 1 of the accompanying drawings. Referring to FIG. 1, the glossmeter disclosed therein comprises an optical system including a light source 1, a slit 2 and a lens 3 and is so designed as to project collimated beams of light onto the surface to be tested at an angle of incidence of 45.degree. or 60.degree.. Depending on the contour of the surface to be tested, the light reflected therefrom may scatter. The glossmeter of the above described construction serves to optically measure the extent to which the image of the slit reflected from the surface to be tested can be visible on a pattern 6, and the distinctness-of-image glossiness measured by the glossmeter shown in FIG. 1 can be calculated from the following equation wherein M represents the maximum value of the light transmitted through a light area on the pattern 6 and m represents the minimum value at a dark area of the same. EQU C=(M-m)/(M+m).times.100%
The larger the value of C, the higher the distinctness-of-image. Conversely, the smaller the value of C, the lower the distinctness-of-image. However, this prior art glossmeter has been found having disadvantages in that the measured values tend to be adversely affected by the ambient rays of light and in that, since the image of the slit projected onto the pattern tends to fail to align with the aperture of a light receptor 7 in the event of the presence of a warp and/or a curve on the surface to be tested, the measurements are adversely affected or are difficult to perform.